Reference may be made to U.S. Pat. No. 5,858,240, Twardowski, Zbigniew, Ulan and Judith issued on Jan. 12, 1999, which describes the removal of sodium chloride from concentrated aqueous solutions where sodium chloride is permeated with simultaneous rejection of other compounds like sodium sulfate to provide a pass solution with high concentration of multivalent ions.
Reference may be made to U.S. Pat. No. 4,702,805, Burkell and Warren, issued on Oct. 27, 1987, which describes an improved method for the control of sulfate concentration in an electrolyte stream in a crystalline chlorate plant, whereby the sulfate is crystallized out. In the production of crystalline sodium chlorate according to U.S. Pat. No. 4,702,805, sodium chlorate is crystallized from sodium chlorate rich liquor. The crystals are removed to provide a mother liquor comprising principally of sodium chlorate and sodium chloride, together with other components, including sulfate and dichromate ions. A portion of the mother liquor is cooled to a temperature to effect crystallization of a portion of the sulfate as sodium sulfate in admixture with sodium chlorate. The crystallized admixture is removed and the resulting spent method liquor is recycled to the electrolytic process.
Reference may be made to a process described in U.S. Pat. No. 4,702,805, wherein the crystallized admixture of sulfate and chlorate obtained from typical commercial liquors may be discolored yellow owing to the unexpected occlusion of a chromium component in the crystals. The discoloration cannot be removed by washing the separated admixture with liquors in which the crystallized sulfate and chlorate are insoluble. It will be appreciated that the presence of chromium in such a sulfate product is detrimental in subsequent utilization of this product and, thus, this represents a limitation to the process as described in U.S. Pat. No. 4,702,805.
Reference may be made to U.S. Pat. No. 4,636,376—Maloney and Carbaugh, issued Jan. 13, 1987, which discloses a process for removing sulfate from aqueous chromate-containing sodium chlorate liquor without simultaneous removal of significant quantities of chromate. The chromate and sulfate-containing chlorate liquor having a pH in the range of about 2.0 to about 6.0 is treated with a calcium-containing material at a temperature range between about 40. degree. C. and 95.degree. C., for time period between 2 and 24 hours to form a sulfate-containing precipitate. The precipitate is predominantly glauberite, Na.sub.2 Ca (SO.sub.4).sub.2. However, the addition of calcium cations requires extra expenditure and effort for the treatment and removal of all excess calcium ions. It is known that calcium ions may form an unwanted deposit on the cathodes which increases the electrical resistance of the cells and adds to operating costs. The calcium ions are removed by means of ion exchange resins.
Reference may be made to U.S. Pat. No. 4,415,677, which describes a method for sulfate ion adsorption. Typically, organic anion exchange resins have a low selectivity for sulfate anions in the presence of a large excess of chlorine ions. The method consists of removing sulfate ions from brine by a macroporous ion exchange resin composite having polymeric zirconium hydrous oxide contained in a vessel. This method has many disadvantages. This method is not economical because the efficiency is low and a large amount of expensive cation exchange resin is required for carrying zirconium hydrous oxide adsorbent. Further, the polymer loaded with zirconium hydrous oxide comes into contact with acidic brine containing sulfate ions, resulting in loss of the adsorbent due to acid-induced dissolution. Soluble zirconyl ions precipitates as hydroxide in the lower portion of the vessel and clogs flow paths.
Reference may be made to U.S. Pat. No. 5,071,563—Shiga et al., issued Dec. 10, 1991, which describes the selective adsorption of sulfate anions from brine solutions using zirconium hydrous oxide slurry under acidic conditions. The ion exchange compound may be regenerated by treatment with alkali.
Reference may be made to Japanese Patent Kokai No. 04321514-A, published Nov. 11, 1992 to Kaneka Corporation, which describes the selective adsorption of sulfate anions from brine solutions using cerium hydroxide slurry under acidic conditions. The ion exchange compound may be regenerated by treatment with alkali.
Reference may be made to Japanese Patent Kokai No. 04338110-A—Kaneka Corporation, published Nov. 25, 1992, which describes the selective adsorption of sulfate anions from brine solutions using titanium hydrous oxide slurry under acidic conditions. The ion exchange compound may be regenerated by treatment with alkali.
The main drawbacks in these and other separation techniques like adsorption, ion exchange etc., which attempt to remove monovalent ions, is the failure of these systems to selectively and economically remove the monovalent ions in a single step from bivalent ions and organic compounds. Substantial portions of the commercially available anion exchange resins are required to sorb sulfate ions. Regeneration of the resins is similarly inefficient because of the need to desorb sulfate ions.
The literature available from the patents describe only the removal of sodium chloride with respect to the Chlor alkali, brine and other industrial solutions. A proper process is absent for the color removal and selective separation of sodium thiocyanate from textile industries. Also the processes described have a very low flow rate which makes them unfeasible on a commercial scale.
Therefore there still remains, a need for an improved, cost-effective, practical method for the removal of sulfate, silica, calcium and iron ions from alkali metal halide solutions, and also organic compounds present if any, particularly from these type of effluents which are used in the spinning of fibres for textile industries.